The present invention relates to radio frequency (RF) amplifiers and particularly to a system for amplifying high power RF signals and to recovering a portion of the signal power normally lost in prior art signal amplifying and combining systems.
Many current communication systems, whether they are used for transmitting analog data or digital data, employ high power RF amplifiers as part of the signal transmission or transponder sections of the system. Frequently, the information to be communicated by these systems is transmitted through the technique of amplitude modulation, AM. Usually, AM refers to full carrier amplitude modulation containing the information to be transmitted (and received) but it should be understood herein that any modulation system which causes the instantaneous composite amplitude of the waveform to be varied in accordance with the information transmitted may be termed an AM system. Single-sideband signals can also be generated by this system using the well known technique of envelope elimination and restoration (EER). Kahn, L. R.: "Single-Sideband Transmission by Envelope Elimination and Restoration", Proc. IRE, 1952, 40, pp. 803-806.
The AM signal to be broadcast may be generated in many ways. In one way, as disclosed in U.S. Pat. No. 4,580,111 to Swanson, an amplitude modulator generates an amplitude modulated carrier signal by selectively combining varying numbers of other carrier signals. Swanson also discloses a circuit which provides plural carrier signals of like frequency and phase where the number of carrier signals provided is dependent upon the number of digital levels chosen. Swanson uses a combiner made up of a plurality of transformers to combine the plural carrier signals and provide a combined signal which is the desired amplitude modulated carrier signal. Other combiner circuits are also disclosed by Swanson, including: voltage addition combiners, current addition combiners, and transmission line combiners. Various of Swanson's embodiments combine carrier signals which are equal in magnitude, carrier signals which are weighted in a binary progression, and, carrier signals which have been frequency modulated.
In another prior art system, disclosed in U.S. Pat. No. 4,804,931 to Hulick, the amplitude modulator comprises a predetermined number of quadrature power hybrid devices configured as combiners and arranged in cascade so that the output of one combiner becomes one of the inputs of its adjacent combiner. A digital decoder responsive to a digital input signal controlled the inputs ports of the combiners. By selectively enabling the RF signal to the various input ports the combined signal a the output of the last of the cascaded combiners represents the desired output signals, which my be fed to an antenna for transmission. The use of quadrature combiners in systems such as those disclosed by Hulick often are inefficient because of losses in the quadrature combiners. (Theoretically 50 percent of the power is lost at one-quarter power out.) For example, if the amplitude of the two signals appearing at the input ports to the combiners are not the same, the output signal will be less than the sum of the two input signals, the balance being reflected back and/or being absorbed by the resistor at the isolated port (i.e., a dummy or reject load). Such systems often have their best efficiency when all the input ports are loaded, a condition which occurs only at the maximum peak power output.
It is accordingly an object of the present invention to provide a novel digital amplitude modulator with improved power efficiency.
It is a further object of the present invention to provide a novel method and apparatus for combining in-phase RF signals.
It is yet another object of the present invention to provide an RF signal combiner in which the output signal tracks the input signal in a linear fashion.
It is still another object of the present invention to provide an RF signal combiner in which portions of the RF signal normally lost due to input signal mismatches can be recovered and used to reduce combiner power losses.
These and other objects of the present invention will become apparent to those skilled in the art from a review of the following specification in conjunction with the drawing figures when accorded a full range of equivalents.